The world is facing an impending energy crisis as the consumption of fossil fuels increases. The dimensions of the crisis include scarcity of fossil fuels as supplies become depleted, escalating costs, confrontations over supplies, pollution and global warming. As more societies modernize in the near future, the magnitude of the crisis will only multiply. It has never been more clear that the world needs to develop and implement new sources of energy.
The most ubiquitous element in the universe, hydrogen, offers unprecedented opportunities for reducing the world's dependence on fossil fuels. Hydrogen is a virtually inexhaustible fuel source and is available from a variety of raw materials including coal, natural gas and hydrocarbons in general, inorganic hydrides and water. Water electrolysis, a method in which water is split into hydrogen gas and oxygen gas, is one attractive method for producing hydrogen. The energy required to effect electrolysis may be obtained from non-fossil fuel sources such as solar energy, wind power, geothermal energy or nuclear energy. In addition to being plentiful and widely available throughout the world, hydrogen is also a clean fuel source. Combustion of hydrogen with oxygen to provide energy produces only water as a by-product and avoids the undesired generation of greenhouse gases and other pollutants. Hydrogen truly is a green energy source.
Having identified hydrogen as a readily available fuel source with desirable combustion properties, realization of a hydrogen based fuel economy requires the development of an infrastructure for making hydrogen accessible to the public. A key element of this infrastructure is a safe and reliable means for storing and delivering hydrogen. The storage of hydrogen in the solid state is the preferred method for storing hydrogen because it avoids the high pressures required for gas phase storage of hydrogen and the low temperatures required for liquid phase storage of hydrogen. The solid state storage of hydrogen is most effectively realized through hydrogen storage alloy materials. A hydrogen storage alloy is a solid state material that is capable of reversibly storing hydrogen, typically in the form of atomic hydrogen.
A conventional hydrogen storage alloy is a metal or metal alloy that includes catalytic sites and hydrogen storage sites. Storage of hydrogen is accomplished through the conversion of hydrogen gas to atomic hydrogen at catalytic sties followed by the binding or retention of hydrogen at hydrogen storage sites. In the hydrogen storage process, a hydrogen storage alloy is exposed to hydrogen gas, which adsorbs onto, diffuses into or otherwise interacts with the hydrogen storage alloy to reach the catalytic sites that convert it to atomic hydrogen. The atomic hydrogen subsequently migrates to a hydrogen storage site, where it is stably retained until the release process is initiated. The release of hydrogen is accomplished by adding thermal energy to the hydrogen storage alloy to free atomic hydrogen from hydrogen storage sites and induce migration of atomic hydrogen to catalytic sites that subsequently effect recombination of atomic hydrogen to form hydrogen gas, which then desorbs, diffuses or otherwise vacates the alloy to provide hydrogen fuel.
An important objective in hydrogen storage is the development of hydrogen storage alloys that are capable of storing large amounts of hydrogen in small volumes with rapid uptake and release of hydrogen. The realization of high hydrogen storage density and rapid kinetics for the storage and release processes requires careful consideration of the chemical and physical factors that contribute to the mechanisms that underlie the hydrogen storage and release processes. The kinetics of the hydrogen storage and release processes are promoted through the presence of a high number of catalytic sites having sufficient activity in a hydrogen storage alloy as well as through the existence of a sufficiently porous surrounding structural matrix to support the catalytic sites. A porous support matrix is desirable because it facilitates access of hydrogen gas to and from the catalytic sites during storage and release, respectively. A porous support matrix is also desirable because it promotes migration of atomic hydrogen to and from hydrogen storage sites during storage and release, respectively.
High hydrogen storage density is promoted through the presence of a high number of hydrogen storage sites that provide sufficient stabilizing interactions to bind or otherwise retain atomic hydrogen. Strong stabilizing interactions, however, precariously compete with the requirements for rapid release kinetics and release at reasonable temperatures. Although the strong binding of hydrogen is conducive to high hydrogen storage density, strongly bound hydrogen is difficult to liberate and is therefore detrimental to the objective of achieving rapid release kinetics. Strongly bound hydrogen can be released at reasonable rates only at high, and oftentimes inconvenient or impractical, temperatures.
Conventional hydrogen storage alloys are metals or metal alloys. Originally, metals or metal alloys prepared by equilibrium melting and cooling methods were used as hydrogen storage alloys. Equilibrium hydrogen storage alloys have structural and bonding properties that are dictated by thermodynamic considerations. Typically, equilibrium hydrogen storage alloys are densely packed with low porosity, a low number of catalytic sites and strongly binding hydrogen storage sites. The dense packing and low porosity are consequences of the metal-metal bonding that is characteristic of metals and metal alloys. With the exception of a thin metal oxide layer at the surface, the metal atoms of metals and metal alloys are exclusively bonded to other metal atoms to form an extended metal-metal bonding scheme that provides a material with closely spaced metal atoms and with strong electronic interactions between metal atoms. The close proximity of metal atoms leads to low porosity as well as to a high electron density between metal atoms that contributes to the typically high stabilization or binding of atomic hydrogen at hydrogen storage sites. The number of catalytic sites is also low because catalysis is primarily a surface phenomenon that occurs at surface irregularities such as dislocation sites, crystal steps, impurities etc. Since these surface irregularities occur unintentionally in equilibrium hydrogen storage alloys, the number of catalytic sites is low and the overall catalytic efficiency is unnecessarily low.
The low porosity, high hydrogen binding energy and low concentration of catalytic sites limits the range of application of conventional hydrogen storage alloys. Since conventional hydrogen storage alloys present a trade-off between the two principal desired characteristics (high hydrogen storage density and rapid kinetics), products and applications of hydrogen storage materials have typically focused on optimizing one of the principle characteristics at the expense of the other. Current hydrogen storage alloys are incapable of providing the high hydrogen storage densities and rapid kinetics at reasonable temperatures required for a wide range of practical applications, including storage and delivery of hydrogen as a fuel for automobiles and other vehicles. The needs of this and other applications requires the development of new hydrogen storage materials based on new and non-conventional chemical and physical paradigms.